signals

Review Wireless Power Transfer Approaches for Medical Implants: A Review

Mohammad Haerinia 1,* and Reem Shadid 2

1 School of Electrical Engineering and Computer Science, University of North Dakota, Grand Forks, ND 58202, USA 2 School of Electrical Engineering Department, Applied Science Private University, Amman 11931, Jordan; [email protected] * Correspondence: [email protected]



Received: 6 September 2020; Accepted: 10 November 2020; Published: 16 December 2020


Abstract: Wireless power transmission (WPT) is a critical technology that provides an alternative for wireless power and communication with implantable medical devices (IMDs). This article provides a study concentrating on popular WPT techniques for IMDs including inductive coupling, microwave, ultrasound, and hybrid wireless power transmission (HWPT) systems. Moreover, an overview of the major works is analyzed with a comparison of the symmetric and asymmetric design elements, operating frequency, distance, efficiency, and harvested power. In general, with respect to the operating frequency, it is concluded that the ultrasound-based and inductive-based WPTs have a low operating frequency of less than 50 MHz, whereas the microwave-based WPT works at a higher frequency. Moreover, it can be seen that most of the implanted receiver’s dimension is less than 30 mm for all the WPT-based methods. Furthermore, the HWPT system has a larger receiver size compared to the other methods used. In terms of efficiency, the maximum power transfer efficiency is conducted via inductive-based WPT at 95%, compared to the achievable frequencies of 78%, 50%, and 17% for microwave-based, ultrasound-based, and hybrid WPT, respectively. In general, the inductive coupling tactic is mostly employed for transmission of energy to neuro-stimulators, and the ultrasonic method is used for deep-seated implants.

Keywords: wireless power transfer; implanted device; inductive link; microwave link; ultrasound link; hybrid link

1. Introduction In recent years, medical progress has evolved with an increased interest in instruments for sensing and controlling the specific functions of the brain. These medical instruments considerably decrease morbidity and improve the standard of life for certain patients. Sensor systems are now quite advanced but providing power to these devices is still a major challenge. The answer to this issue is using wireless power transmission (WPT) technologies for a range of biomedical implants. WPT is a secure and appropriate energy supply for recharging biosensors and electrical implanted devices as well as for data communication in these specific applications. WPT comprises two main methods: near-field and far-field transmission. The region is considered near-field if it satisfies two conditions: First, the distance between the transmitter and receiver coil (d) should be less than one wavelength (λ) at the operating frequency (d < λ), and second, the largest dimension of the transmitter coil (D) should be less than λ/2. In contrast, for far-field D > λ/2. Moreover, 2 the near and far fields are defined in terms of the Fraunhofer distance (dF = 2D /λ); if the conditions dF >> D and dF >> λ are satisfied, the region is considered far-field.

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There are three major ways to accomplish a near-field WPT: (1) capacitive coupling based on electric fields; (2) inductive coupling based on magnetic fields; and (3) magnetic resonant inductive coupling, which include a resonant circuit in transmitter and receiver coils. Far-field WPT is also Signals 2020, 3 FOR PEER REVIEW 2 known as microwave coupling. Hybrid wireless power transmission (HWPT) includes both far-field andThere near-field are three WPT. major ways to accomplish a near-field WPT: (1) capacitive coupling based on electricThe fields; biomedical (2) inductive implants coupling are based intended on magn toetic be fields; used and for (3) biological magnetic resonant studies, inductive therapy, and medical coupling,diagnostics. which Novel include biological a resonant materials circuit in also transmitter provide and additional receiver biocompatibilitycoils. Far-field WPT and is also efficiency, as well known as microwave coupling. Hybrid wireless power transmission (HWPT) includes both far-field as reduced expenses. Implantable medical devices (IMDs) can be classified into two primary categories and near-field WPT. basedThe on biomedical their methodologies implants are intended for the transmissionto be used for ofbiological power. studies, Transfer therapy, mechanisms and medical such as inductive diagnostics.coupling, Novel optical biological charging, materials and ultrasound also provide are additional included biocompatibi in the firstlity category. and efficiency, The secondas category wellis split as reduced into two expenses. subsections: Implantable batteries, medical such devic ases lithium;(IMDs) can and be naturalclassified harvesting,into two primary including biofuel categoriescell, thermoelectricity, based on their methodologies piezoelectricity, for the electrostatic, transmission and of power. electromagnetic Transfer mechanisms [1]. Various such WPTas techniques inductiveare reviewed coupling, in theoptical literature. charging, Forand instance,ultrasound the are ultrasoundincluded in the and first inductive category. couplingThe second methods were categoryevaluated is split by Taallainto two et al.subsections: [2] and Shadid batteries, et al.such [3 ],as respectively.lithium; and natural In this harvesting, paper, common including WPT approaches biofuel cell, thermoelectricity, piezoelectricity, electrostatic, and electromagnetic [1]. Various WPT for IMDs, including inductive coupling, microwave, and ultrasound, are studied. HWPT systems, techniques are reviewed in the literature. For instance, the ultrasound and inductive coupling methodsa mixture were of evaluated two different by Taalla methods, et al. [2] are and also Shad reviewed.id et al. [3], respectively. In this paper, common WPT approaches for IMDs, including inductive coupling, microwave, and ultrasound, are studied. HWPT2. Diff systems,erent Approaches a mixture of two for different a Wireless methods, Power are Transfer also reviewed. System The lifespan of IMDs is limited to battery capabilities. Patient pain and the danger of infection are 2. Different Approaches for a Wireless Power Transfer System the major development concerns in implantable medical systems because using implanted batteries canThe cause lifespan diseases of IMDs [4]. is Therefore,limited to battery the WPT capabilities. link is Patient a safer pain option and tothe power danger biomedicalof infection implants [4]. are the major development concerns in implantable medical systems because using implanted Improving WPT techniques and efficiency will enable rechargeable batteries to be employed for IMDs batteries can cause diseases [4]. Therefore, the WPT link is a safer option to power biomedical implantsrather than [4]. Improving non-rechargeable WPT techniques batteries, and which effici usuallyency will have enable a greater rechargeable weight batteries and volume to be and a shorter employedperiod of for e ffIMDsectiveness rather than compared non-rechargeable to rechargeable batteries, batteries. which usually Medical have a implants greater weight like implantedand spinal volumecord stimulators and a shorter can period use aof rechargeable effectiveness batterycompared to to improve rechargeable their batteries. capability Medical and reduce implants overall costs [5]. likeLately, implanted there spinal has been corda stimulators great interest can use in thea rechargeable usage of WPT battery for to medicalimprove applications.their capability Theand development reduceof implantable overall costs electronic [5]. Lately, devices there has in biologicalbeen a great systems interest hasin the made usage it easierof WPT to for use medical this technology for applications.powering variousThe development IMDs, such of implantable as biological electr sensors,onic devices pacemakers, in biological and systems neurostimulator, has made it working in a easier to use this technology for powering various IMDs, such as biological sensors, pacemakers, and range of power from a few microwatts to a few watts. In Figure1, the power ranges of common IMDs neurostimulator, working in a range of power from a few microwatts to a few watts. In Figure 1, the powerare illustrated ranges of common [1,6]. The IMDs WPT are systemsillustrated for [1,6 the]. The neurostimulator WPT systems for and the neurostimulator the pacemaker and are discussed in thedetail pacemaker in [7–13 are]. discussed in detail in [7–13].

Drug Pump Neurostimulator Cardiac Defibrillator Biosensor Cochlear Implant Pacemaker Retinal Implant VA D *

μW mW W *Ventricular Assist Device

FigureFigure 1. Power 1. Power ranges of ranges implantable of implantable medical devices medical (IMDs). devices (IMDs).

There are reliability problems with the classic wireless power links. An option that facilitates the growth of a number of bio-implants is the use of CMOS processes. In this procedure, the standard CMOS is included with the implanted receiver. This reduces the cost, improves productivity, and provides compatibility and reliability of prototypes [14,15]. The usage of CMOS for WPT systems is presented in [15–23]. A backward communication unit transmits the information to an external data communicator using modulation. Typically, FSK [24], PSK [25], ASK [26], OOK [27], LSK [28], PPSK [29], Signals 2020, 1 211

QMPM [30], QPSK [31], and impedance modulation [32] have been used for data communication units in medical applications. Long-term RF and microwave exposure are dangerous. The device layout must comply with the associated safety regulations to protect patients from electromagnetic radiation damage. It is possible to evaluate maximum permissible exposure (MPE) in environments for electromagnetic field intensity by assessing the specific absorption rate (SAR). IEEE Standard Basis C95.1 expresses SAR limitation. According to IEEE 1992 standard, the maximum SAR value must be below 1.6 W/kg for any 1 g of the body tissue and below 0.08 W/kg for the whole body. Nevertheless, the maximum SAR limitation is 4 W/kg for every 10 g of tissue of body parts such as hands, feet, ankles, and wrists, as per IEEE 2005 standard. The authors proposed an arrangement to decrease the SAR and improve safety for inductive WPT systems [33]. They designed a multi-transmitter configuration consisting of an array of symmetric resonant elements. The array will significantly decrease the amount of electric field generated by the fed loop and thus reduce the electromagnetic exposure of the biological tissues. Mainly, determining SAR can be achieved by using numerical techniques and empirical models using fabricated tissue phantoms [4,34–38]. In [39], the body tissue was used as the power transfer channel. According to this technique, medical electrodes are attached on the body surface to supply power to a miniaturized implant with a differential input. The maximum SAR value was studied in [40,41]. The empirical results can be obtained in vivo [8,42,43], using a living organism, or in vitro [44–46], outside of a living organism. To mimic the biological effects of human body tissue, the phantom is very popular among researchers in this field. The material type and amount needed for muscle phantom fabrication is summarized and measured in [47]. The tissue’s electromagnetic properties play an important role in the design of implantable devices. An assessment of variation in tissue electromagnetic properties was provided by Bocan et al. [48]. The recent reports on tissue electromagnetic properties are depicted in Table1.

Table 1. Summary of different approaches in analyzing tissue electromagnetic properties.

Reference Year Tissue Frequencies Models/Methods [49] 2020 In vivo, ex vivo - FEM * 50 MHz, 300 MHz, 700 MHz, [50] 2019 Muscle, fat, skin FDTD ** and 900 MHz Measured properties, [51] 2019 Body (0.5–26.5) GHz Cole–Cole [52] 2018 Brain, liver 200–1600 Hz Measured properties [53] 2018 Muscle, fat, skin 915 MHz and 2 GHz Measured properties Bottcher–Bordewijk [54] 2017 Blood, liver, fat, brain 10 kHz–10 MHz model, measured properties Muscle, bladder, Measured properties, [55] 2016 128 MHz cervix Cole–Cole [56] 2016 Body/14 tissues 2.1 GHz, 2.6 GHz FDTD [57] 2016 Head (0.75–2.55) GHz Phantom/ FEM [58] 2016 Muscle 500 MHz–20 GHz Fricke [59] 2015 Eye/6 tissues (0.9–10) GHz FDTD [60] 2015 Skin (0.8–1.2) THz FEM [61] 2014 Eye, head/14 tissues (0.9–5.8) GHz FDTD [62] 2010 Head - FEM Measured properties, [63] 2009 Head/16 tissues 50 MHz–20 GHz FDTD 900 MHz, 1800 MHz, [64] 2006 Eye, head/15 tissues FDTD 2450 MHz [65] 2004 Body 400 MHz, 900 MHz, 2400 MHz Visible human, FDTD [66] 2004 Body/51 tissues 30 MHz–3 GHz FDTD [67] 2002 Head/10 tissues 900 MHz, 1800 MHz Visible human, FDTD * Finite element method. ** Finite-difference time-domain. SignalsSignals 20202020,, 13 FOR PEER REVIEW 2124 Signals 2020, 3 FOR PEER REVIEW 4

2.1. Inductive-Based Wireless Power Transfer 2.1.2.1. Inductive-BasedInductive-Based WirelessWireless Power Power Transfer Transfer Inductive coupling is the process of transferring power by connecting a source that is generating Inductive coupling is the process of transferring power by connecting a source that is generating a varyingInductive magnetic coupling field is theto a process primary of coil transferring which is power usually by located connecting outside a source the thatbody is tissue. generating Then, a a varying magnetic field to a primary coil which is usually located outside the body tissue. Then, varyingbased on magnetic Faraday field′s law, to athe primary voltage coil is induced which is usuallyacross the located receiver outside secondary the body coil, tissue. which Then, is usually based based on Faraday′s law, the voltage is induced across the receiver secondary coil, which is usually onimplanted Faraday 0insides law, the voltagebody tissues. is induced Figure across 2 below the receiverillustrates secondary this principle. coil, which is usually implanted insideimplanted the body inside tissues. the body Figure tissues.2 below Figure illustrates 2 below this illustrates principle. this principle.

PrimaryPrimary CoilCoil TransmitterTransmitter coilcoil andand locatedlocated outsideoutside thethe humanhuman bodybody

SecondarySecondary CoilCoil ImplantedImplanted receiverreceiver insideinside bodybody tissuestissues

Figure 2. Inductive coupling principle. Figure 2. Inductive coupling principle. Figure 2. Inductive coupling principle. The amount of induced voltage in implanted coils (V ) is given by the following equation is) The amount of induced voltage in implanted coils ( ) is given by the following equation The amount of induced voltage in implanted coils ( isZ given by the following equation dΦ → → Vis== −N ==jN ωΦ = = jNωµ H.ds (1)(1) = −− dt = = .· (1) where N is the number of turns, is the operating angular frequency, is the magnetic flux linkage, wherewhereN Nis is the the numbernumber ofof turns,turns, ωis is the the operating operating angular angular frequency, frequency,Φ isis the the magnetic magnetic flux flux linkage, linkage, and is the permeability of transfer medium. According to Equation (1), coupling between coils andandµ is is the the permeability permeability of of transfer transfer medium. medium. According According to to Equation Equation (1), (1), coupling coupling between between coils coils depends mainly on the amount of between the primary (transmitter) and secondary (implanted) dependsdepends mainlymainly onon thethe amountamount ofofΦ between between the the primary primary (transmitter) (transmitter) and and secondary secondary (implanted) (implanted) coils. Thus, when the distance between the transmitter and the implanted receiver is decreased the coils.coils. Thus,Thus, whenwhen thethe distancedistance betweenbetween thethe transmittertransmitter andand thethe implanted implanted receiverreceiver isis decreased decreased the the amount of coupled magnetic flux will increase. amountamount of of coupled coupled magnetic magnetic flux flux will will increase. increase. Furthermore, the amount of transferred power using inductive coupling could be increased by Furthermore,Furthermore, the the amount amount of of transferred transferred power power using using inductive inductive couplingcoupling couldcould bebe increasedincreased by by adding a capacitor for resonance. The simplified diagram of the resonant inductive WPT circuit is addingadding aa capacitor capacitor for for resonance. resonance. TheThe simplifiedsimplified diagramdiagram ofof thethe resonantresonant inductiveinductive WPTWPT circuitcircuit isis shownshown inin FigureFigure3 3.. L1is is the the transmitter transmitter coil coil inductor inductor that that is is located located outside outside the the body body tissues, tissues, and andL2 is shown in Figure 3. is the transmitter coil inductor that is located outside the body tissues, and is the implanted receiver coil inductor, often with the rest of the implant electronics. Coil windings theis the implanted implanted receiver receiver coil coil inductor, inductor, often often with with the rest the of rest the of implant the implant electronics. electronics. Coil windings Coil windings have have parasitic capacitance and resistance associated with them, which are shown as symmetric parasitichave parasitic capacitance capacitance and resistance and resistance associated associated with them, with which them, are which shown are as shown symmetric as symmetric elements (elementsRs1, Rs2), and(, ( Cs1,),C sand2). Capacitors (, ). CCapacitorsT and CR are added and to theare circuitadded totoform the circuit an LC resonanceto form an with LC elements (, ), and (, ). Capacitors and are added to the circuit to form an LC Lresonance1 and L2, respectively.with and RL,is respectively. the load resistance. is the load resistance. resonance with and , respectively. is the load resistance.

Figure 3. Circuit model of magnetic resonant inductive coupling. Figure 3. Circuit model of magnetic resonant inductive coupling. Figure 3. Circuit model of magnetic resonant inductive coupling.

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The highest efficiency and voltage gain are achieved when both LC tanks are tuned at the operating frequency of the link ωo = 1/ √L1C1 = 1/ √L2C2, where C1 and C2 are a combination of the lumped capacitor and the parasitic capacitance of the transmitter and implanted coils, respectively. The delivered power is transferred between the transmitter and the implanted coils through mutual inductance (M). M is related to the coupling coefficient (k) according to

M k = (2) √L1L2

The quality factors for the transmitter (Q1), receiver (Q2), and load (QL) circuits are calculated as follows: ωoL1 ωoL2 RL Q1 = , Q2 = , QL = (3) Rs1 Rs2 ωoL2

The total efficiency, ηind, is calculated according to Equation (4), as derived in [3]:

2 k Q1Q2 1 ηind = (4) 1 + k2Q Q + Q2 × 1 + QL 1 2 QL Q2

More details on how to derive the efficiency of inductive links can be found in [68]. In general, the efficiency increases at high delivered power. However, different efficiencies have been achieved for the same transmitted power depending on the system design. Inductive coupling is a common and efficient way to transfer data and power into implantable medical instruments, including cardiac pacemakers, implantable cardioverter defibrillators, recording devices, neuromuscular stimulators, and cochlear and retinal implants. When the development of an inductive link using a power amplifier is applied, the output power depends on the operating frequency and the distance range. The bandwidth to support data communication and reasonable efficacy for power transfer, insensitivity to misalignments, and biocompatibility are needed for a robust inductive link for medical implants [69]. In general, hundreds of kilohertz to a few megahertz is the operating frequency, and the size of the implanted coil is between several millimeters and a few centimeters. As the frequency increases, the electromagnetic wavelength becomes more commensurate with the coil dimension and the space between the coils. In this stance, the radiative and non-radiative components are part of the electromagnetic waves. Biological tissue also creates significant problems for the propagation of electromagnetic fields and dilutes the electrical field, thus affecting the efficiency of the inductive link [33]. According to Faraday’s induction law, increasing the size of coils and the number of turns boosts inductive link efficiency [33]. When the transmitting coil and the receiving coil have the same size, the maximum coupling is achievable. Although, in practice, the implanted coil is significantly smaller than the transmitting coil [70]. Mainly, the inductive-based WPT system is used for medical devices such as brain and spinal cord stimulators. Lyu et al. [8] have developed a stimulator that occupies an area of 5 mm 7.5 mm and operates × at the resonant frequency of 198 MHz while having a 14 cm distance from the transmitter, which is located outside of the body. The stimulator gets the energy that has already been stored by a switched capacitor and releases the energy as an output stimulus once the voltage reaches a threshold. The control unit utilizes positive feedback to trigger the circuit, so no stimulation control circuit block is needed. An in vivo experiment was performed to demonstrate the performance of the stimulator. Two electromyography (EMG) recording electrodes were implanted into the gastrocnemius muscle of a rat while the ground electrode was attached to the skin. A free-floating neural implant, which is insensitive to the location, is provided as an inductive link in [10] for wireless energy transmission. The authors have created prototypes of floating implants for precise measurements. The system works with a power transfer efficiency of 2.4% at 60 MHz and provides 1.3 mW power to the implant 14–18 mm away from the transmitter. Their coil link is stable against the lateral and angular misalignments of the floating implants if the coils continue to have the Signals 2020, 1 214

Signals 2020, 3 FOR PEER REVIEW 6 high-Q resonator. The extra heat produced by the resonator coil also does not exceed safety limits. RecentRecent works works of ofthe the inductive inductive WPT WPT scheme scheme are are evalua evaluatedted and and presented presented in inTable Table 2.2 The. The panel panel consists consists of ofprinted printed and and 3D 3D coils. coils. Printed Printed coils coils maintain maintain acceptable acceptable performance performance under under lateral lateral malalignment malalignment andand are are reliable reliable for for implants implants [4]. [4 ].

2.2.2.2. Microwave-Based Microwave-Based Wireless Wireless Power Power Transfer Transfer AnotherAnother way way to toefficiently efficiently transmit transmit power power wirelessly wirelessly over over long long distances distances in in the order of meters to to kilometerskilometers is is microwave microwave power power transmission. transmission. Figure Figure4 illustrates 4 illustrates the external the external and implanted and implanted antennas 0 antennasbehavior.′ behavior. It should It beshould noted be that noted up-link that isup-link defined is whendefined the when implanted the implanted antenna actsantenna as a transmitteracts as a transmitterand the external and the antenna external act antenna as a receiver, act as awhereas receiver, down-link whereas down-link is vice versa. is vice versa.

Figure 4. Microwave wireless power transmission (WPT) principle. Figure 4. Microwave wireless power transmission (WPT) principle. Assuming far-field WPT, the budget power link as discussed in [71] can be described as follows: Assuming far-field WPT, the budget power link as discussed in [71] can be described as follows:  C   C  Link margin (dB/Hz) = N N (/) = o Link −− o Required (5) Eb = P + G L + G N 10 log B +G G ta tg f ra 0 No r c d (5) − − − − − = + − + − − − 10 + − where Pta . is the transmitted power in dBW, Gtg is the transmitting antenna gain in dBi, L f is the path loss in dB, Gra is the receiving antenna gain in dBi, No is the noise power density in dB/Hz, Br is the bit where is the transmitted power in dBW, is the transmitting antenna gain in dBi, is the rate in kb/s, Gc is the coding gain in dB, and Gd is the fixing deterioration in dB. path loss in dB, is the receiving antenna gain in dBi, is the noise power density in dB/Hz, The path loss can be calculated through the equation below, taking into consideration that the is the bit rate in kb/s, is the coding gain in dB, and is the fixing deterioration in dB. free-space signal strength reduces with the increase in distance between the transmitter and receiver: The path loss can be calculated through the equation below, taking into consideration that the free-space signal strength reduces with the increase in distance ! between the transmitter and receiver: 4πd L = 20 log (6) f 4λ =20log (6) where d is the distance between the transmitter and the receiver and λ is the wavelength. Considering where d is the distance between the transmitter and the receiver and is the wavelength. the impedance mismatch losses, Considering the impedance mismatch losses,   L = 10 log 1 r2 (7) impedance − − where r isTable the appropriate2. Existing inductive-based reflection coe ffiWPTcient. approaches Both L f andfor implantableLimpedance are power considered applications. for more accurate evaluation. The received powerOutput by the receiver can be calculatedActive asTransmitter follows: Receiver Efficiency Reference Year Frequency Power Range Dimension Dimension (%) Pr(mW)= P ta + Gtg + Gra L (mm)L e(mm)p (mm) (8) − f − impedance − [72] 2020 915 MHz - 1.93 40–50 - 30 × 30 [73] 2020 5.8 GHz 0.01 1.2 × 10 1 - 0.116 × 0.116 [7] 2019 430 MHz 1000 - 45 - 4.5 × 3.6 [74] 2019 13.56 MHz 57–447 5.7–44.7 20–50 75 × 75 20 × 30 [34] 2019 434 MHz 31.62 0.68 10 20 × 20 1.6 × 1.6

[8] 2018 198 MHz 1000 - 140 d = 30.5 d = 4.9 60,300, 2.12, 3.88, [41] 2018 - 12 d = 17.2, 24, 26 d = 4 330 MHz 1.68

[75] 2018 2, 4 MHz 126 25 6 d = 35 d = 20

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where ep is the polarization mismatch loss between the transmitter and the receiver. Equation (8) can be also described as follows:

2 GtgGraλ  2 2 Pr = 1 S11 1 S22 ep Pt (9) (4πd)2 − | | − | | ×

In practice, the received power value for microwave design can be extracted from the value of

2 Pr S21 = (10) | | Pt

Table 2. Existing inductive-based WPT approaches for implantable power applications.

Output Active Transmitter Receiver Efficiency Reference Year Frequency Power Range Dimension Dimension (%) (mW) (mm) (mm) (mm) [72] 2020 915 MHz - 1.93 40–50 - 30 30 5 × [73] 2020 5.8 GHz 0.01 1.2 10− 1 - 0.116 0.116 [7] 2019 430 MHz 1000× - 45 - 4.5 × 3.6 [74] 2019 13.56 MHz 57–447 5.7–44.7 20–50 75 75 20 × 30 [34] 2019 434 MHz 31.62 0.68 10 20 × 20 1.6 × 1.6 × × [8] 2018 198 MHz 1000 - 140 doutT = 30.5 doutR = 4.9 60,300, 2.12, 3.88, d = 17.2, [41] 2018 - 12 outT d = 4 330 MHz 1.68 24, 26 outR [75] 2018 2, 4 MHz 126 25 6 doutT = 35 doutR = 20 [9] 2018 1.3 GHz 3981 - 5 doutT = 10 doutR = 0.2 [35] 2018 39.86 MHz 115 47.2 - doutT = 63.9 doutR = 21.56 [76] 2018 432.5 MHz 1.05 13.9 10 - - [77] 2018 430 MHz - - 60 30 30 10 10 × × [78] 2018 3 MHz 772.8 38.79 5–15 doutT = 45.2 doutR = 36.4 [11] 2017 13.56 MHz 18 7.7, 11.7 10 d 30 d = 10 outT ≈ outR [40] 2016 50 MHz 0.0657 0.13 10 doutT = 21 doutR = 1 [19] 2014 8.1 MHz 29.8~93.3 47.6~65.4 12~20 doutT = 30 doutR = 20 [21] 2019 12.85 MHz - 75.8 - 30.0 29.6 30.0 29.6 × d ×= 1.75 [79] 2019 1–100 MHz - - 15 - outR dinR = 0.50 [42] 2018 433 MHz 0.1, 1, 4, 10 0.87 600 - doutR = 10 [29] 2017 13.56 MHz 100 - 5–15 d = 25 d = 16 ≤ outT outR [10] 2017 60 MHz 1.3 2.4 16 doutT = 45 doutR = 1.2 [37] 2016 20 MHz 2.2 1.4 10 doutT = 20,28 doutR = 1 [43] 2016 40 MHz - 2.56 70 doutT = 100 doutR = 18 [30] 2015 2 MHz 1450 27 80 doutT = 140 doutR = 65 [80] 2015 800 kHz 30 w 95 20 doutT = 70 doutR = 34 [81] 2012 742 kHz - 85 0–50 doutT = 38 doutR = 16.5

It should be noted that some amount of transmitted power will be dissipated in tissue due to radiation and coupling into the body [82]. The implant placement depth plays a key role in the amount of lossy power led by the body tissue. The present-day challenges for this technique include the minimization of energy loss, protecting both humans and animals against exposure to excessive microwave radiation, and the reconfiguring of a wireless transmission system resulting from modifications such as a shifting in range between transmitter and receiver [83]. Microwave WPT can transfer a high amount of power between the transmitter and the receiver circuits. However, it is worth mentioning that human tissues cause problems for the propagation of electromagnetic fields and dilute the electrical field. Therefore, the reflection caused by the lossy mediums reduces the overall power transfer efficiency. Pacemaker implantation is a popular method of curing people with cardiac insufficiency. However, the lifetime of the pacemaker is restricted to the lifespan of the battery and the installation of a subcutaneous pocket [13]. Asif et al. [13] built a rectenna-based leadless pacemaker prototype. For energy transmission to the implanted unit, a wearable transmitting antenna range was fabricated to evaluate the system0s efficiency through Vivo electrocardiogram (ECG) outcomes. The authors assert that the calculations of SAR are within the limits suggested by IEEE and claim that the proposed leadless pacing method is safer, and eliminates the battery, lead, and Signals 2020, 1 216 device pocket. Zada et al. [84] provided a miniaturized implantable antenna with three frequency bands (902–928, 2400–2483.5, and 1824–1980 MHz) operating at the industrial, scientific, and medical (ISM) band and at the midfield band. A capsule-shaped and a flat type antenna were fabricated with a volume of 647 mm3 and 425.6 mm3, respectively. This triple band antenna was complemented withSignals microelectronics, 2020, 3 FOR PEER REVIEW sensors, and batteries for stimulation in different applications. The system8 was examined in different tissues, including the scalp, heart, colon, large intestine, and stomach. al. [85] took advantage of a microwave-based WPT technique to charge deep medical implants like Asif et al. [85] took advantage of a microwave-based WPT technique to charge deep medical implants cardiac pacemakers. Their novel wideband numerical model (WBNM) was to provide an RF power like cardiac pacemakers. Their novel wideband numerical model (WBNM) was to provide an RF source of a leadless pacemaker while using a metamaterial-based antenna operating at 2.4 GHz. They power source of a leadless pacemaker while using a metamaterial-based antenna operating at 2.4 GHz. used tissue simulating liquid (TSL) mimicking the human body to prove the performance of their They used tissue simulating liquid (TSL) mimicking the human body to prove the performance of their design for implantable applications. A wireless powering technique was introduced by Ho et al. [82], design for implantable applications. A wireless powering technique was introduced by Ho et al. [82], which overcomes the difficulty of miniaturizing the power source via adaptive electromagnetic which overcomes the difficulty of miniaturizing the power source via adaptive electromagnetic energy energy transport. This method is designed for micro-implants like micro-electromechanical system transport. This method is designed for micro-implants like micro-electromechanical system sensors sensors and opto-elements. Figure 5 shows the wireless electrostimulator inserted into the lower and opto-elements. Figure5 shows the wireless electrostimulator inserted into the lower epicardium epicardium of a rabbit. Recent works of microwave-based WPT systems are reviewed and shown in of a rabbit. Recent works of microwave-based WPT systems are reviewed and shown in Table3. Table 3.

FigureFigure 5. 5.Photograph Photograph of of the the electrostimulator electrostimulator insertedinserted in the lower epicardium of of a a rabbit rabbit via via open- open- chestchest surgery surgery [82 [82].].

TableTable 3. 3.Existing Existing microwave-based microwave-based WPT WPT approachesapproaches forfor implantableimplantable powerpower applications.

OutputOutput ActiveActive TransmitteTransmitterr ReceiveReceiverr EfficiencyEfficiency ReferenceReference YearYear Frequency Frequency PowerPower RangeRange DimensionsDimensions DimensionsDimensions (%)(%) (mW)(mW) (mm)(mm) (mm)(mm) (mm)(mm) [86[86]] 2020 2020 1.47 1.47 GHz GHz 6.7 6.7 0.67 0.67 50 50 6 × 6 6 6 - - × 0.4030.403 GHz, GHz, [87[87]] 20202020 -- - - 30–350 30–350 - - 9.5 9.5 × 9.59.5 2.442.44 GHz GHz × 1.64 GHz, [88] 2019 1.64 GHz, 3.56 - 32, 1.1 - 14 15 14 15 [88] 2019 3.56 GHz - 32, 1.1 - 14 × 15× 14 × 15× GHz [13] 2019 954 MHz 10 65 110 - 10 12 × [13] 2019 0.915, 954 MHz 1.9, 10 65 110 - 10 × 12 [84] 2018 0.915, 1.9, 0.398 - 4.5 - 7 6 [84] 2018 2.45 GHz 0.398 - 4.5 - 7 × 6× [38] 2018 4002.45 MHz GHz 19, 82 - 1, 3, 6, 12, 15 d = 18 1 1 outT × [89] 2018 280 MHz 44 -1, 3, 6, 3 30 80 - [38] 2018 400 MHz 19, 82 - d = 18× 1 × 1 2280, 600, 12, 15 [90] 2017 2.45 GHz - 1000–4000 - doutR = 63.6 [89] 2018 280 MHz 240, 44 96 - 3 30 × 80 - [91] 2014 2.4 GHz - 15–78 10–100 63 39 50 63 39 50 2280, × × × × 1000– - [90] 2017 2.45 GHz 600, - d = 63.6 4000 2.3. Ultrasonic-Based Wireless Power Transfer240, 96 [91] 2014 2.4 GHz - 15–78 10–100 63 × 39 × 50 63 × 39 × 50 The ultrasound imaging is a well-known tool for evaluating patients’ physiological and pathological conditions. In the passive ultrasonic recorder, the backscattered echo is derived from 2.3. Ultrasonic-Based Wireless Power Transfer the reaction of biological tissue0s acoustic properties to sound waves. Additionally, the acoustic emissionThe canultrasound be used forimaging supplying is a well-known energy wirelessly tool for in evaluating the active biologicalpatients’ physiological environment and [45]. Thepathological ultrasonic-based conditions. WPT In system the passive has a ultrasonic transmitter recorder, converting the backscattered electrical energy echo to is ultrasonicderived from energy, the andreaction a receiver of biological converting tissue back′s acoustic the ultrasonic properties energy to sound to electrical waves. energy. Additionally, the acoustic emission can be used for supplying energy wirelessly in the active biological environment [45]. The ultrasonic- based WPT system has a transmitter converting electrical energy to ultrasonic energy, and a receiver converting back the ultrasonic energy to electrical energy. The basic model of the implantable ultrasonic coupling WPT system is shown in Figure 6. The transmitting transducer powered by the transmitting module sends the ultrasonic waves, and the ultrasonic energy is transmitted to the receiving transducer through the human tissue. The receiving transducer converts the collected ultrasonic energy into electrical power. Accordingly, power is

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The basic model of the implantable ultrasonic coupling WPT system is shown in Figure6. The transmitting transducer powered by the transmitting module sends the ultrasonic waves, and the Signals 2020, 3 FOR PEER REVIEW 9 ultrasonicSignals 2020 energy, 3 FOR PEER is transmitted REVIEW to the receiving transducer through the human tissue. The receiving9 transducer converts the collected ultrasonic energy into electrical power. Accordingly, power is delivered to the implantable device through the receiving power module. The receiving power delivereddelivered to to the the implantable implantable devicedevice throughthrough thethe receivingreceiving power module. module. The The receiving receiving power power processing module mainly includes a voltage-stabilizing circuit and a rectifier circuit. processingprocessing module module mainly mainly includes includes aa voltage-stabilizingvoltage-stabilizing circuit circuit and and a a rectifier rectifier circuit. circuit.

Figure 6. The basic model of the implantable ultrasonic coupling WPT system. FigureFigure 6. 6.The The basicbasic modelmodel of the implantable ultrasonic ultrasonic coupling coupling WPT WPT system. system. A modelA model of of the the ultrasonic ultrasonic principle principle of of WPT WPT inin thethe transducertransducer of the transmitter transmitter and and receiver receiver is is A model of the ultrasonic principle of WPT in the transducer of the transmitter and receiver is shownshown in Figurein Figure7[92 7 ];[92]; the the radiated radiated sound sound power powerP Pis is given given byby thethe belowbelow equation: equation: shown in Figure 7 [92]; the radiated sound power P is given by the below equation: 1  ( )2 11 Z a (J ka√ +) (11) = 1 √ 2 2 2 2 √ l ++d (11) =P = πρocouaa dl (11) 0 l where is the density of the medium, is the sound velocity of the medium, is the amplitude, where is the density of the medium, is the sound velocity of the medium, is the amplitude, wherea is ρtheo is sound the density source of radius the medium, of the circularco is the plane sound A, = velocity is the of the wavenumber medium, u ofa is a thesound amplitude, field, isa is a is the sound source radius of the circular plane A, =ω is the wavenumber of a sound field, is the sound source radius of the circular plane A, k = is the wavenumber of a sound field, d is the the distance along the z-axis, and ( ) is the cfirst-ordero Bessel function.   the distance along the z-axis, and (1 ) is the first-order Bessel function. distance along the z-axis, and J1 ka is the first-order Bessel function. √l2+d2 x x

A Q(r,θ) A h θ h Q(r, ) dS r θ l B dS r θ l B d z d z

y y Figure 7. The field model of the ultrasonic coupling wireless transmission system [92]. Figure 7. The field model of the ultrasonic coupling wireless transmission system [92]. The ultrasonic-basedFigure 7. The field WPT model system of the is ultrasonic an effective couplin methodg wireless for medical transmission applications system [92]. such as cardiac defibrillatorsThe ultrasonic-based and deep brain WPT stimulators system is (DBSs)an effective [93]. method A mode for of medical clinical applications therapy is asuch stimulation as cardiac of The ultrasonic-based WPT system is an effective method for medical applications such as cardiac excitabledefibrillators tissue forand di deepfferent brain disorders, stimulators such (DBSs) as Parkinson’s [93]. A mode disease, of clinical urinary therapy incontinence, is a stimulation and heart of defibrillators and deep brain stimulators (DBSs) [93]. A mode of clinical therapy is a stimulation of arrhythmia.excitable tissue The traditional for different stimulus disorders, techniques such as useParkinson’s percutaneous disease, cables urinary to transport incontinence, electricity and heart to the excitablearrhythmia. tissue The for traditional different disorders, stimulus techniquessuch as Parkinson’s use percutaneous disease, urinarycables to incontinence, transport electricity and heart to electrodes. The classical techniques are dangerous because they can cause infection [12]. arrhythmia.the electrodes. The The traditional classical stimulus techniques techniques are dangerous use percutaneous because they cables can cause to transport infection electricity[12]. to the electrodes.The ultrasound- The classical or inductive-based techniques are daWPTngerous is an because interesting they solution can cause for infection this application. [12]. The advantageThe ultrasound- of ultrasound or inductive-based compared to magneticWPT is anresonance interesting and solution induction for couplingthis application. is that theseThe advantage of ultrasound compared to magnetic resonance and induction coupling is that these

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SignalsThe 2020, ultrasound-3 FOR PEER REVIEW or inductive-based WPT is an interesting solution for this application.10 The advantage of ultrasound compared to magnetic resonance and induction coupling is that these methodsmethods are are restricted restricted to to a a short transfer distan distance,ce, misalignment misalignment issues issues may may occur occur [94], [94], and and the the magneticmagnetic field field intensityintensity mustmust bebe under under specified specified limitations limitations for for the the safety safety of of the the body body exposure. exposure. In theIn theultrasonic ultrasonic method, method, the the operating operating frequency frequency needs needs to be to changedbe changed according according to sound to sound radiation radiation and andpressure pressure distribution distribution to obtain to obtain the optimum the optimum energy transitionenergy transition situation situation [93]. In the [93]. range In ofthe frequencies range of frequenciesindividuals individuals hear, Kim et hear, al. [ 94Kim] have et al. developed [94] have developed an implantable an implantable pressure-sensing pressure-sensing system driven system by drivenmechanical by mechanical vibration. vibration. The pressure The inductor pressure has inductor a planar has coil a planar with a coil center with of ferritea center in of which ferrite their in whichdistance their diff distanceers from differs the involved from the stress. involved An implantablestress. An implantable pressure sensor pressure prototype sensor prototype was designed, was designed,as shown as in Figureshown8 ,in and Figure examined 8, and inexamined vitro and inin vitro vivo .and The in acoustic vivo. The receiver acoustic is a receiver piezoelectric is a piezoelectriccantilever and cantilever charges a capacitorand charges by converting a capacitor sound by converting vibration harmonics sound vibration into electrical harmonics energy. into The electricalstored electric energy. charge The stored is discharged electric across charge an is LCdisc tankharged with across an inductor an LC tank sensitive with toan pressure inductor during sensitive the toperiod pressure that during the cantilever the period is not that shaking. the cantilever is not shaking.

FigureFigure 8. 8. AnAn implantable implantable pressure-sensing pressure-sensing system. system.

SongSong et etal. al.[95] [investigated95] investigated omnidirectional omnidirectional ultrasonic ultrasonic powering poweringfor deep implantable for deep microdevices. implantable Whenmicrodevices. testing the When omnidirectionality testing the omnidirectionality and outcome of the and power outcome transmission of the power under transmission the acousticunder Food andthe acousticDrug Administration Food and Drug (FDA) Administration regulations, (FDA) the piez regulations,oelectric devices the piezoelectric with distinct devices geometries with distinct were examined.geometries The were receivers examined. were The able receivers to produce were power able to in produce a range power of milliwatts in a range with of milliwattsa matched with load a locatedmatched 200 load mm located away from 200 mm them. away The from receiv them.ers had The symmetric receivers hadgeometry symmetric of 2 × geometry 2 × 2 mm of and 2 were2 2 × × insensitivemm3 and were to misalignment. insensitive to Recent misalignment. works of Recent ultrasonic-based works of ultrasonic-based WPT systems are WPT shown systems in Table are shown 4. in Table4. Table 4. Existing ultrasonic-based WPT approaches for implantable power applications. Table 4. Existing ultrasonic-based WPT approaches for implantable power applications. Output Active Transmitter Receiver Efficiency Reference Year Frequency PowerOutput RangeActive DimensionTransmitter DimensionReceiver (%) Efficiency Reference Year Frequency Power Range Dimension Dimension (mW) (%) (mm) (mm) (mm) (mW) (mm) (mm) (mm) [96] 2020 700 kHz - - 200 - d = 10 [97] [96]2017 20201 MHz 700 kHz 0.1 - - - 85 200d = 0.55 - doutR = - 10 [97] 2017 1 MHz 0.1 - 85 d = 0.55 - [98] 2017 1.8 MHz - 2.11 30 d = outT10.8, 15.9 d = 1.1, 1.2 d = 10.8, d = 1.1, [16] [98]2016 20171 MHz 1.8 MHz 0.184 - - 2.11 - 30 outT - outR - 15.9 1.2 [99] 2016 1 MHz - 25 3–7 d = 8 - [16] 2016 1 MHz 0.184 - - - - [100] 2015 280 kHz 2.6 18 18 d = 20 d = 20 [99] 2016 1 MHz - 25 3–7 doutT = 8 - [101][100 ]2015 20153.4 MHz 280 kHz 0.001 2.6 - 18 100 18 d -outT = 20 doutR = - 20 [17][ 101]2015 201530 MHz 3.4 MHz 0.1 0.001 - - <100 100 - -d - = 0.7,1 [18] [17] 2014 2015 1 MHz 30 MHz 28 0.1 1.6 - 105 <100 29.6 × 72 - * doutR 1 =× 50.7,1 ** [102] 2013 1.07 MHz - 45 - - - [103] 2011 1.2 MHz 100 50 - d = 44 - [46] 2011 2.3 MHz 0.3 - 30–400 d = 8 - [12] 2011 1 MHz 23 - 120 - d = 8 [104] 2010 35 kHz 1.23 - 70 - d = 7

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Table 4. Cont.

Output Active Transmitter Receiver Efficiency Reference Year Frequency Power Range Dimension Dimension (%) (mW) (mm) (mm) (mm) [18] 2014 1 MHz 28 1.6 105 29.6 72 * 1 5 ** × × [102] 2013 1.07 MHz - 45 - - - [103] 2011 1.2 MHz 100 50 - doutT = 44 - [46] 2011 2.3 MHz 0.3 - 30–400 d = 8 - ≈ outT [12] 2011 1 MHz 23 - 120 - doutR = 8 [104] 2010 35 kHz 1.23 - 70 - doutR = 7 [105] 2010 673 kHz 1000 27 40 - - Signals 2020[106, 3 ]FOR PEER 2003 REVIEW 100 kHz 5400 36 40 -11 - [25] 2002 1 MHz 2100 20 40 - - [105] [26]2010 673 2001 kHz 1 MHz1000 27 - 40 20 30 - - - - [106] * Width2003 and total100 kHz length of 485400 symmetric elements 36 of the 40 spherical transducer - array. ** Active - area of a single [25] element2002 of the1 flat MHz transducer 2100 array. 20 40 - - [26] 2001 1 MHz - 20 30 - - * Width and total length of 48 symmetric elements of the spherical transducer array. ** Active area of 2.4. Hybrid Wireless Power Transfer a single element of the flat transducer array. A hybrid wireless power transmission (HWPT) system is a combination of two common methods 2.4.working Hybrid Wireless as a unit Power system. Transfer The inductive WPT system uses magnate fields to transfer power, whereas the capacitiveA hybrid WPT wireless system power uses electrictransmission fields. (HWPT) The capacitive system is WPT a combination approach hasof two advantagescommon compared methodswith the working inductive as a unit one. system. First, The there inductive is no WPT eddy system current uses loss magnate and, fields second, to transfer it uses power, a lightweight and whereaslow-cost the coupler. capacitive However, WPT system the capacitive uses electric method fields. is The limited capacitive to small WPT power approach transfer has and two short distance advantages compared with the inductive one. First, there is no eddy current loss and, second, it uses because of the small coupling capacitor. When the transfer distance is in several hundred millimeters, a lightweight and low-cost coupler. However, the capacitive method is limited to small power transferthe coupling and short capacitor distance isbecause usually of inthe the small picofarad coupling range. capacitor. The When voltages the transfer across thedistance coupling is in plates of the severalcoupler, hundred which millimeters, could be improved the coupling with capacitor double-sided is usually transformers in the picofarad or various range. The compensation voltages topologies across(such the as coupling double-sided plates of LC the and coupler, Z-source), which are could usually be improved hundreds with of double-sided times the input transformers voltages to enhance orthe various system compensation power level. topologies Considering (such as that double-sided the high LC voltage and Z-source), stressed are in theusually coils hundreds of the inductive unit ofcould times bethe fullyinput used voltages as ato driving enhance voltage the system for power the capacitive level. Considering coupler, that combining the high bothvoltage systems can be stresseddone as in athe hybrid coils of system. the inductive Therefore, unit could it is importantbe fully used to as take a driving advantage voltage of for the the inductive capacitive and capacitive coupler, combining both systems can be done as a hybrid system. Therefore, it is important to take hybrid system to achieve higher power for HWPT. advantage of the inductive and capacitive hybrid system to achieve higher power for HWPT. A Ahybrid hybrid method method includes includes inductive inductive power transf powerer transferand capacitive and capacitivepower transfer, power as shown transfer, in as shown in FigureFigure 9.9 .

Primary Capacitor

I C Primary Coil P M1 IS V V Vin L 1 2 L Secondary Coil 1P 1S V out Secondary Capacitor

CM2

Figure 9. Drawing and diagram of hybrid wireless power transmission (HWPT). Figure 9. Drawing and diagram of hybrid wireless power transmission (HWPT). As shown above, the inductive link will generate the alternating magnetic field through coils, As shown above, the inductive link will generate the alternating magnetic field through coils, whichwhich provides provides the thetransmission transmission medium. medium. Furthermore, Furthermore, those currents those will currents produce will high produce voltages high voltages onon the the transfer transfer coil coilbecause because of the of self-inductance, the self-inductance, whereas whereasthe capacitive the capacitivelink system linkrequires system highrequires high voltagesvoltages to toproduce produce the electric the electric field for field the for capa thecitive capacitive coupler. coupler.Accordingly, Accordingly, the produced the voltage produced voltage onon coils coils of ofthe the inductive inductive link system link system can becan used be for used the capacitive for the capacitive system. A system.combined A system combined to system to construct a hybrid electric power transfer is established in [107]. and present the port voltages as follows:

= + (12a)

= + (12b) The efficiency of the inductive link is given by the same equation as Equation (4). Whereas the power transfer from the capacitive link of the hybrid system is given by Equation (13):

=− ( −) (13)

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construct a hybrid electric power transfer is established in [107]. V1 and V2 present the port voltages as follows: V1 = jωL1PIP + jωMIS (12a)

V2 = jωL1SIS + jωMIP (12b) The efficiency of the inductive link is given by the same equation as Equation (4). Whereas the power transfer from the capacitive link of the hybrid system PCH is given by Equation (13):

3  2  P = ω CIPI M L L (13) CH − S − 1P 1S where IP and IS are the RMS currents in primary and secondary coils. L1P and L1S represent the self-inductance of the transmitting and receiving side coils, respectively. M is the mutual inductance. The advantage of a hybrid system is that it occupies less space than separate units. In this case, there is a capability of having different operating bands, more alternatives, and backup. In [108], the authors proposed a coupled WPT–Power Line Communication (PLC) system that consists of a two-coil resonator system. A four-port system has been designed with two ports dedicated to power transfer and the other two to data transfer. Capacitors have been used to tune the power channel to the desired frequency and improve the matching of the filtering stage to the coupled inductors. A hybrid inductive-based and a microwave-based WPT system are also presented in [109]. One of the current challenges for wireless transfer for small sensors is to minimize the system size. Haerinia et al. [109] decreased the size of the compact system, at the same time implementing multi-functionality. This goal was obtained by designing an antenna with 14 mm 15 mm dimensions × and having 20 mm 20 mm dimensions for the hybrid system including the antenna and coil. The coils × operating frequency was 510 MHz and the antennas worked at 2.48 GHz and 4.66 GHz. Meng et al. [110] developed a hybrid inductive-ultrasonic WPT link to power biomedical implants over bone, air, and tissue. They optimized cascaded inductive and ultrasonic links for WPT applications. The hybrid link was designed for an air–tissue medium to operate at 1.1 MHz with a power transmission efficiency of 0.16%. Recent works related to the HWPT system are shown in Table5. The receiver dimension should be as small as possible to make it more convenient for implant applications. In case the receiver dimension is larger than 40 mm, such design can be printed on a flexible material substrate such as Kapton to facilitate the surgery procedures [111]. It is worth mentioning that rotational/lateral misalignment and bending are two conditions that may happen because of changes in the implanted antenna location or the person’s movement. Therefore, the bending of a flexible substrate and the misalignment effect must be investigated precisely [112].

Table 5. Existing HWPT approaches for implantable power applications.

Output Active Transmitter Receiver Efficiency Reference Year Frequency Power Range Dimension Dimension Methods (%) (mW) (mm) (mm) (mm) 510 MHz, Inductive and [109] 2019 2.48 GHz, 0.0004 3.7, 2.2, 1 20–60 d = 39.75 d = 20 outT outR Microwave 4.66 GHz 1.9, 2.6, Inductive and [113] 2018 4 MHz 500, 53, 53 >30,15,30 - d = 40,83,83 0.98 outR Capacitive 13.56 MHz, 415 MHz, 10, 0.5, Inductive and [114] 2020 - 15–110 d = 79.6 d = 31.5 905 MHz, 4.6, 6.5 outT outR Microwave 1300 MHz 13.56 MHz/910 Inductive and [115] 2017 - 17 16 d = 83.2 d = 24.2 MHz outT outR Microwave Inductive and [110] 2017 1.1 MHz - 0.16 60 d = 100 d = 15 outT outR Ultrasonic Inductive and [116] 2012 200 kHz 8 1 70 d = 39 d = 33 outT outR Ultrasonic Signals 2020, 1 221

3. Consideration for Design of Medical implants and Related Regulations Developments in wireless technology for medical devices are elevating the provision of healthcare with lower expenses. Wireless telecommunications can be used for both wearable and implantable applications, such as DBSs, tracking of vital signs, measuring biological parameters, and cardiac rhythm control. The main advantage of wireless technology compared to landline networks is that the patient is not required to be linked to a certain location by cables [117]. Despite advances in biomedical implants such as the pacemaker, cochlear implant, and nerve stimulator, these devices need to be improved in terms of miniaturization, the biocompatibility of materials, sources of electric charge, and wireless communication. To develop an effective IMD, the doctor, the patient, and the technician must collaborate in collecting coherent initial information about different aspects of the device. In particular, the user’s satisfaction, the doctor’s technical priorities, and the workability of the model are necessary to be considered in the design process [118]. There are important factors for designing medical implants. Since an electric device is implanted inside the human body, the organisms around the device may react to it. To avoid such an issue, the device should be made up of or coated by biocompatible materials. Moreover, the medical implants should have appropriate packaging to isolate components of the device from body tissue. Another factor is the structure of the design itself. Before the design, enough data should be collected from patients, engineers, and previous designs, along with their advantages and drawbacks [118]. The United States0 medical devices market is regulated by three different organizations: the Federal Communications Commission (FCC), the Food and Drug Administration (FDA), and the Centers for Medicare and Medicaid Services (CMS). Wireless medical instruments can be classified into two categories: short-range, such as inductive implants and medical body area networks, and long-range, such as wireless medical telemetry (WMTS). According to the FCC, short-range technology sends data to local receivers and long-range technology sends user data to a remote spot [117]. The FDA’s mission is to check if the proposed medical devices guarantee the factors of safeness and effectiveness for patient usage. The FDA divides medical devices into three classifications based on the risk factor. Class I includes the lowest-risk devices, and without FDA prior authorization, medical devices in this class may be advertised. The medical devices using wireless technologies are usually considered in Class II. The highest-risk medical devices fall under Class III and clinical trials are mandatory to get FDA approval. The FCC and FDA must permit before wireless medical devices can be marketed in the United States. It is worth mentioning that the FDA and FCC have distinct criteria, and one agency0s authorization does not simply ensure the other0s consent [117]. The designers of medical implants are currently dealing with challenges in materials, output power, size miniaturization, the efficiency of the wireless link, and cybersecurity [118]. There are different types of cyber-attacks, including theft of protected health information and execution of fraudulent device commands, which require appropriate cybersecurity mechanisms [119]. It is crucial to have a broad perspective of different aspects of wireless techniques before choosing the tactic for any specific applications. Figure 10 shows a comparison between the collected research papers for different approaches: the maximum dimension of a receiver, power transfer efficiency, and frequency. From Figure 10a,b, it can be seen that most of the ultrasound-based WPT systems work at a low operating frequency, ranging from a few hundred kHz up to few MHz. Most likely, inductive-based WPT systems mostly operate at a low operating frequency but range at a higher frequency compared to ultrasound-based and reach up to a few hundred MHz. Moreover, it can be concluded that operating frequency for most of the research based on inductive WPT lay below 50 MHz. Furthermore, 13.56 MHz is the interest frequency for the majority of the inductive WPT research field. On the contrary, most of the collected papers’ research related to microwave-based WPT located at a higher operating frequency; compared to both ultrasound-based and inductive-based WPT, it ranged from hundreds of MHz to hundreds of GHz. On the other hand, it was noticed that the operating frequency of hybrid wireless power transmission (HWPT) varies from low to high frequency. This is due to the fact that HWPT Signals 2020, 3 FOR PEER REVIEW 13

technician must collaborate in collecting coherent initial information about different aspects of the device. In particular, the user’s satisfaction, the doctor’s technical priorities, and the workability of the model are necessary to be considered in the design process [118]. There are important factors for designing medical implants. Since an electric device is implanted inside the human body, the organisms around the device may react to it. To avoid such an issue, the device should be made up of or coated by biocompatible materials. Moreover, the medical implants should have appropriate packaging to isolate components of the device from body tissue. Another factor is the structure of the design itself. Before the design, enough data should be collected from patients, engineers, and previous designs, along with their advantages and drawbacks [118]. The United States′ medical devices market is regulated by three different organizations: the Federal Communications Commission (FCC), the Food and Drug Administration (FDA), and the Centers for Medicare and Medicaid Services (CMS). Wireless medical instruments can be classified into two categories: short-range, such as inductive implants and medical body area networks, and long-range, such as wireless medical telemetry (WMTS). According to the FCC, short-range technology sends data to local receivers and long-range technology sends user data to a remote spot [117]. The FDA’s mission is to check if the proposed medical devices guarantee the factors of safeness and effectiveness for patient usage. The FDA divides medical devices into three classifications based on the risk factor. Class I includes the lowest-risk devices, and without FDA prior authorization, medical devices in this class may be advertised. The medical devices using wireless technologies are usually considered in Class II. The highest-risk medical devices fall under Class III and clinical trials are mandatory to get FDA approval. The FCC and FDA must permit before wireless medical devices can be marketed in the United States. It is worth mentioning that the FDA and FCC have distinct criteria, and one agency′s authorization does not simply ensure the other′s consent [117]. The designers of medical implants are currently dealing with challenges in materials, output power, size miniaturization, the efficiency of the wireless link, and cybersecurity [118]. There are

Signalsdifferent2020, 1 types of cyber-attacks, including theft of protected health information and execution222 of fraudulent device commands, which require appropriate cybersecurity mechanisms [119]. It is crucial to have a broad perspective of different aspects of wireless techniques before choosing the tactic for any comprisesspecific aapplications. combination Figure of more 10 than shows one a methodcomparison to transfer between power the collected wirelessly, research such as papers inductive-based for different andapproaches: capacitive-based. the maximum dimension of a receiver, power transfer efficiency, and frequency.

Signals 2020, 3 FOR PEER REVIEW (a) 14

(b)

FigureFigure 10. A comparison of didifferentfferent approaches.approaches. ( a) Maximum dimension of receiverreceiver versusversus frequency;frequency; ((bb)E) Efficiencyfficiency versusversus frequency.frequency.

FigureFrom Figure 10a presents 10a,b, it the can maximum be seen that dimensions most of the of ultrasound-based the receiver versus WPT the systems operating work frequency. at a low Inoperating general, frequency, it can be noticedranging that from most a few of hundred the receivers’ kHz up dimensions to few MHz. are Most less thanlikely, 30 inductive-based mm for all the WPT-basedWPT systems methods. mostly operate This is at due a low to the operating fact that freq miniaturizationuency but range of at the a higher implanted frequency device compared is highly recommendedto ultrasound-based in the medicaland reach field up to to simplify a few thehundred surgery MHz. procedures Moreover, and toit becan more be comfortableconcluded that for operating frequency for most of the research based on inductive WPT lay below 50 MHz. Furthermore, 13.56 MHz is the interest frequency for the majority of the inductive WPT research field. On the contrary, most of the collected papers’ research related to microwave-based WPT located at a higher operating frequency; compared to both ultrasound-based and inductive-based WPT, it ranged from hundreds of MHz to hundreds of GHz. On the other hand, it was noticed that the operating frequency of hybrid wireless power transmission (HWPT) varies from low to high frequency. This is due to the fact that HWPT comprises a combination of more than one method to transfer power wirelessly, such as inductive-based and capacitive-based. Figure 10a presents the maximum dimensions of the receiver versus the operating frequency. In general, it can be noticed that most of the receivers’ dimensions are less than 30 mm for all the WPT- based methods. This is due to the fact that miniaturization of the implanted device is highly recommended in the medical field to simplify the surgery procedures and to be more comfortable for the patients. Besides, it is shown in Figure 10a that the receiver dimensions of HWPT have a larger size compared to the other methods of WPT. Moreover, the receiver dimensions for most of the collected research on ultrasound-based and microwave-based WPT are below 20 mm. Additionally, a cluster of biomedical implant receivers using the inductive-based technique have a maximum dimension of less than 20 mm and operate at a considerably lower frequency compared to the microwave-based technique with almost the same size. Figure 10b presents the efficiency percentage versus the operating frequency. In general, it can be interpreted that the maximum power transmission efficiency is achieved via inductive-based WPT, whereas the minimum power transmission efficiency is achieved via HWPT; on the other hand, the maximum efficiencies achieved are 78% and 50% for microwave-based and ultrasound-based WPT, respectively. In the same way, we conclude from Figure 10b that the operating frequency is much higher for microwave-based WPT compared to inductive-based WPT, which has almost the same efficiency.

Signals 2020, 1 223 the patients. Besides, it is shown in Figure 10a that the receiver dimensions of HWPT have a larger size compared to the other methods of WPT. Moreover, the receiver dimensions for most of the collected research on ultrasound-based and microwave-based WPT are below 20 mm. Additionally, a cluster of biomedical implant receivers using the inductive-based technique have a maximum dimension of less than 20 mm and operate at a considerably lower frequency compared to the microwave-based technique with almost the same size. Figure 10b presents the efficiency percentage versus the operating frequency. In general, it can be interpreted that the maximum power transmission efficiency is achieved via inductive-based WPT, whereas the minimum power transmission efficiency is achieved via HWPT; on the other hand, the maximum efficiencies achieved are 78% and 50% for microwave-based and ultrasound-based WPT, respectively. In the same way, we conclude from Figure 10b that the operating frequency is much higher for microwave-based WPT compared to inductive-based WPT, which has almost the same efficiency.

4. Conclusions This research has evaluated and discussed a survey of the following popular methods for wirelessly transferring power into IMDs: (1) inductive-based WPT, (2) microwave-based WPT, (3) ultrasound-based WPT, and (4) hybrid wireless power transmission (HWPT). In this research, the power delivered in the reviewed works to medical implants varied from a few µW to 5.4 W, with distance ranges from 1 mm to 4 m and maximum efficiency of up to 95%. Based on collected papers’ research, it was concluded that ultrasound-based and inductive-based WPT works at low operating frequency (less than 50 MHz), whereas the microwave-based WPT typically works at a higher frequency. On the other hand, the HWPT could be found at a low or high operating frequency, depending on the combination used. It can be seen that the receiver dimension was less than 30 mm for all the WPT-based methods. Furthermore, HWPT had a bigger receiver size. The maximum power transfer efficiency was conducted via inductive-based WPT. Based on collected papers, the value of achievable maximum efficiencies were 95%, 78%, 50%, and 17% for inductive-based, microwave-based, ultrasound-based, and HWPT, respectively. This paper provides a perspective on different WPT approaches for biomedical applications by investigating the significant works in this field. Additionally, some points for designing effective IMDs and related commercial rules and regulations are presented.

Author Contributions: Conceptualization, M.H. and R.S.; methodology, M.H. and R.S.; investigation, M.H. and R.S.; resources, M.H.; writing—original draft preparation, M.H.; writing—review and editing, M.H. and R.S.; visualization, M.H. and R.S.; supervision, M.H.; project administration, M.H. All authors have read and agreed to the published version of the manuscript. Funding: The financial support of the School of Electrical Engineering and Computer Science, University of North Dakota, is acknowledged. Conflicts of Interest: The authors declare no conflict of interest.

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